Hair can be damaged by natural or artificially induced processes. The most important type of damage here can be oxidative damage.
The natural processes can for example comprise a combined (e.g. simultaneous) action of UV light and oxygen (O2) on the hair.
The artificially induced processes can for example comprise an application of hair colourants (also designated as colourations), dying blond and/or producing a permanent wave. In particular, hair colourants play a prominent role in hair cosmetics.
In this case, in addition to desired cosmetic effects such as, for example, a lightening of the hair, severe damage to the hair can also occur, for example, when using oxidants.
The damage process here can take place causatively by oxidation of amino acids, for example, oxidation of the amino acids cystine and cysteine which occur very frequently in hair to give cysteic acid. Cystine can form intermolecular disulphide bridges (also designated as S-S bridges) in the hair so that the cysteine is extremely important for the mechanical stability of the hair.
The oxidation of these bridges to give cysteic acid can destroy the mechanical stability of the hair and in many applications can even result in a complete breaking of the hair. A loss of mechanical stability can result in an intensified swelling of the hair in water.
However, properties of the hair which were already previously macroscopically perceptible, e.g. sensible, for example, a surface roughness can be negatively influenced.